1. Field of the Invention
This invention relates to electrostatic discharge protection circuits. More particularly, the invention relates to electrostatic discharge protection circuits used to protect field effect transistor integrated circuits, especially those with features of one micron or smaller.
2. Description of the Prior Art
Integrated circuits employing field effect devices, commonly termed MOS integrated circuits, are susceptible to electrostatic discharge. Given the decreasing size of circuit features resulting from improving process technology, static electricity generated by daily activity alone can destroy or substantially harm many MOS circuits. The circuits most susceptible to damage are usually finished circuits which have been packaged, but not yet installed in a finished product. Once installed, other means can protect the chip from damage.
An electrostatic discharge typically occurs when the circuit is touched by an individual handling the circuit before installation; when a static discharge occurs as the packaged circuit slides on its pins across another surface; or more generally, whenever the circuit is exposed to static electricity. Overall, damage from electrostatic discharges is the cause of over half of the devices returned by customers.
One traditional method for protecting integrated circuit devices comprised of field effect transistors from electrostatic discharge is to use diodes. These diodes are coupled between the input paths of the circuit and the pins to which the power supplies are connected. With electrostatic discharge events of one polarity the diodes are forward-biased, and with discharges of the opposite polarity they are reverse-biased. Normally the discharge that causes the diodes to become reverse-biased is the more problematic, because voltages and power surges seen by the internal logic circuits are higher than for the forward-biased case. Other methods used for protecting MOS circuits from electrostatic discharge are almost always variations on the diode clamping system described above.
The traditional methods described above usually function satisfactorily for circuits with large features. However, as the features of integrated circuits approach one micron and less, lower voltages than those which damage larger features can destroy the diodes and the circuit. The reverse junction breakdown voltage of known diodes is too high to protect these new integrated circuits.